1. Field of the Invention
The invention relates to an energy stabilization method for a lithography laser, and particularly to a method for compensating energy transients at a workpiece in a laser output energy control algorithm.
2. Discussion of the Related Art
Excimer lasers are typically used for industrial applications in combination with processing systems. Such systems can be lithography waver scanners or TFT-annealing-systems for example. Typically these laser systems have internal detectors to measure certain laser parameters. These laser parameters can include pulse energy, pulse energy dose over a certain number of pulses, spatial beam profile in a plane of the processing system, temporal pulse duration or one or more additional laser beam parameters.
The excimer laser system used for those applications has normally on board metrology measurement tools which allow the measurement and stabilization of similar parameters of the laser beam as pulse energy, pulse energy dose and so on. Under real conditions the detectors used in the on board metrology and the detectors used in the processing tool may measure and/or exhibit different characteristics. For example, detectors in the processing tool may be polarization sensitive, while detectors in the laser system may be not sensitive to the polarization of the incident light.
In addition, the path of the beam between the laser beam detector and the detector of the processing tool may include such optics as an aperture, such that an intensity difference between the detectors may vary with the beam profile or beam width. Such variance may occur as a result of a beam divergence varying over time such as may depend on the structure of the burst sequencing. For example, a beam divergence may be greater during or after prolonged burst periods as compared to periods of reduced exposure due to heating of optics or changes in the gas mixture temperature and/or composition.
In view of the above, a lithography laser system for incorporating with a semiconductor processing system is provided including a discharge chamber filled with a laser gas including molecular fluorine and a buffer gas, multiple electrodes within the discharge chamber and connected with a discharge circuit for energizing the laser gas, a resonator including the discharge chamber for generating a laser beam, and a processor. The processor runs an energy control algorithm and sends a signal to the discharge circuit based on the algorithm to apply electrical pulses to the electrodes so that the laser beam exiting the laser system has a specified first energy distribution over a group of pulses. The energy control algorithm is based upon a second energy distribution previously determined of a substantially same pattern of pulses as the group of pulses having the first energy distribution. The second energy distribution is determined for the laser beam at a location after passing the beam through beam shaping optical elements of the semiconductor processing system while a value of the energy of the laser beam exiting the laser system is maintained at an approximately constant first energy.
In further view of the above, the laser beam energy exiting the laser is determined to change according to the second energy distribution as a result of passing through said beam shaping optical elements. The beam exiting the laser has the substantially constant first energy over the group of pulses being transformed to the beam after the beam shaping optical elements having the second energy distribution over the group of pulses. The first energy distribution may be determined substantially as the approximately constant first energy divided by the second energy distribution, within a steady-state linear energy reduction multiple of the second energy distribution between the beam exiting the laser and the beam after the beam shaping optics. Alternatively, for small overshoot, the first energy distribution may be determined as the first energy minus the second energy distribution.
The first energy distribution may thus preferably have the form:
Elaser(t)=E0/KF(t), where K is a constant, F(t) is a function of time and the second energy distribution has a form E(t)=E1xc2x7Kxc2x7F(t), where E1 is a desired energy of the beam after the beam shaping optics, and E0 is the first energy of said beam exiting the laser, which first energy E0 is sufficient to produce the desired energy E1 after the beam shaping optics when the laser is operating in steady state. The constant K may be E0/E1. The function F(t) may be Aexe2x88x92(t/xcfx84), wherein KA is a magnitude of a transient overshoot, t is a time and xcfx84 is a time constant. In addition, the first energy distribution may be used in the energy control algorithm for a predetermined time after a long burst pause, after which the laser beam exiting said laser is maintained at the substantially constant first energy. Alternatively, for small overshoot, the first energy distribution may take the alternative form Elaser(t)=E0xe2x88x92KF(t), and the second energy distribution may have the form E(t)=E1+KF(t).
In further view of the above, a method for stabilizing a laser beam energy at a location after beam shaping optical elements of a semiconductor fabrication system is provided including generating a laser beam and passing the beam through the beam shaping optical elements. A first energy distribution is determined of the laser beam at a location after passing through the beam shaping optical elements over a burst pattern including a group of laser pulses while the laser beam is maintained at an approximately constant first energy. The processor is programmed with an energy control algorithm based on the first energy distribution.
Electrical pulses are applied to discharge electrodes of the laser system based on the energy control algorithm such that the laser beam exiting the laser system has a second energy distribution over the burst pattern including the group of pulses, such that an energy of the beam at the location after the beam shaping optics is controlled to be substantially a desired constant second energy.
The laser beam energy exiting the laser may be determined to change according to the second energy distribution as a result of passing through the beam shaping optical elements. The beam exiting the laser may have the substantially constant first energy over the group of pulses and be transformed to the beam after the beam shaping optical elements and have the second energy distribution over the group of pulses.
The applying step may include determining the first energy distribution substantially as the approximately constant first energy divided by the second energy distribution, within a steady-state linear energy reduction multiple of the second energy distribution between the beam exiting the laser and the beam after the beam shaping optics. Alternatively, for small overshoot, the first energy distribution may be determined as the first energy minus the second energy distribution.
The first energy distribution may have the form:
Elaser(t)=E0/KF(t), wherein K is an constant, F(t) is a function of time, and the second energy distribution has a form E(t)=E1xc2x7Kxc2x7F(t), wherein E1 is the second energy and E0 is the first energy, which first energy E0 is sufficient to produce the desired energy E1 after the beam shaping optics when the laser is operating in steady state. F(t) may be Aexe2x88x92(t/xcfx84), wherein KA is a magnitude of a transient overshoot, t is a time and xcfx84 is a time constant. Alternatively, for small overshoot, the first energy distribution may take the alternative form Elaser(t)=E0xe2x88x92KF(t), and the second energy distribution may have the form E(t)=E1+KF(t).